CYP1B1 is a member of the dioxin-inducible CYP1 gene family which also includes CYP1A1 and CYP1A2 as described by Sutter et al. (J. Biol. Chem., May 6; 269(18):13092-9, 1994). CYP1B1 is a heme-thiolate mono-oxygenase enzyme that is capable of metabolizing and activating a variety of substrates including steroids, xenobiotics, drugs and/or prodrugs. CYP1B1 protein is expressed to a high frequency in a wide range of primary and metastatic human cancers of different histogenic types and is not expressed or at negligible levels in normal tissue. (see, e.g.: McFadyen M C, Melvin W T and Murray G I, “Cytochrome P450 Enzymes: Novel Options for Cancer Therapeutics”, Mol Cancer Ther., 3(3): 363-71, 2004; McFadyen M C and Murray G I, “Cytochrome P450 1B1: a Novel Anticancer Therapeutic Target”, Future Oncol., 1(2): 259-63, 2005; Sissung T M, Price D K, Sparreboom A and Figg W D, “Pharmacogenetics and Regulation of Human Cytochrome P450 1B1: Implications in Hormone-Mediated Tumor Metabolism and a Novel Target for Therapeutic Intervention”, Mol. Cancer Res., 4(3): 135-50, 2006).
More specifically, CYP1B1 has been shown to be expressed in bladder, brain, breast, colon, head and neck, kidney, lung, liver, ovarian, prostate and skin cancers, without being expressed in the corresponding normal tissue. For example, Barnett, et al., in Clin. Cancer Res., 13(12): 3559-67, 2007, reported that CYP1B1 was over-expressed in glial tumours, including glioblastomas, anaplastic astrocytomas, oligodendrogliomas and anaplastic oligodendrogliomas, but not unaffected brain tissue; Carnell, et al., in Int. J. Radiat. Oncol. Biol. Phys., 58(2): 500-9, 2004, reported that CYP1B1 was over-expressed in prostate adenonocarcinomas, but not in matched normal prostate tissue; Carnell, et al., 2004 (ibid.) also showed that CYP1B1 is expressed in (n=22, 100%) of bladder carcinomas; Downie, et al., in Clin. Cancer Res., 11(20): 7369-75, 2005 and McFadyen, et al., in Br. J. Cancer, 85(2): 242-6, 2001, reported increased expression of CYP1B1 in primary and metastatic ovarian cancer, but not in normal ovary tissue; and Gibson, et al., in Mol. Cancer Ther., 2(6): 527-34, 2003, and Kumarakulasingham, et al., in Clin. Cancer Res., 11(10): 3758-65, 2005, reported that CYP1B1 was over-expressed in colon adenocarcinomas as compared to matched normal tissue.
Several studies have shown that CYP1B1 is over-expressed in breast cancer as compared to matched normal tissue (see, e.g.: Murray G I, Taylor M C, McFadyen M C, McKay J A, Greenlee W F, Burke M D and Melvin W T, “Tumor-Specific Expression of Cytochrome P450 CYP1B1”, Cancer Res., 57(14): 3026-31, 1997; Haas S, Pierl C, Harth V, Pesch B, Rabstein S, Bruning T, Ko Y, Hamann U, Justenhoven C, Brauch H and Fischer H P, “Expression of Xenobiotic and Steroid Hormone Metabolizing Enzymes in Human Breast Carcinomas”. Int. J. Cancer, 119(8): 1785-91, 2006; McKay J A, Murray G I, Ah-See A K, Greenlee W F, Marcus C B, Burke M D and Melvin W T, “Differential Expression of CYP1A1 and CYP1B1 in Human Breast Cancer”, Biochem. Soc. Trans., 24(2): 327S, 1996).
Everett, et al., in J. Clin. Oncology, 25: 18S, 2007, reported that CYP1B1 was over-expressed in malignant melanoma and disseminated disease but not in normal skin. Chang, et al., in Toxicol. Sci., 71(1): 11-9, 2003, reported that CYP1B1 protein is not present in normal liver but Everett, et al., 2007 (ibid.) confirmed CYP1B1 over-expression in melanoma stage IV metastasis to the liver but not in the adjacent normal liver tissue.
Greer, et al., in Proc. Am. Assoc. Cancer Res., 45: 3701, 2004, reported that CYP1B1 was over-expressed during the malignant progression of head and neck squamous cell carcinoma but not in normal epithelium.
McFadyen, et al., in Br. J. Cancer, 91(5): 966-71, 2004, detected CYP1B1 in renal carcinomas but not in corresponding normal tissue.
Murray, et al., 2004 (ibid.) used immunohistochemistry to show over-expression of CYP1B1 in lung cancer cells as compared to normal lung tissue. Su, et al., in Anti-Cancer Res., 2, 509-15, 2009, used immunohistochemistry to show over-expression of CYP1B1 in advanced stage IV non-small cell lung cancer compared to earlier stages of the disease.
It is evident from the numerous disclosures cited above that CYP1B1 expression is characteristic of a range of different cancers and other proliferative conditions, and that CYP1B1 expression may be used to define such a range of cancers and other conditions. As normal (non-cancerous) cells do not express significant levels of CYP1B1, it may also be reasonably expected that compounds that exhibit cytotoxicity in cells expressing CYP1B1, but are substantially non-cytotoxic in normal cells, would have utility as targeted anti-cancer agents in cancers characterized by CYP1B1 expression. By “targeted” is meant that such compounds could be delivered systemically and would only be activated in the presence of cancerous cells expressing CYP1B1, remaining substantially non-toxic to the rest of the body.
Furthermore, a number of cytochrome P450 enzymes are known to metabolise and detoxify a variety of anticancer drugs. McFadyen, et al. n (Biochem Pharmacol. 2001, Jul. 15; 62(2): 207-12) demonstrated a significant decrease in the sensitivity of docetaxel in cells expressing CYP1B1 as compared with non-CYP1B1 expressing cells. This finding indicates that the presence of CYP1B1 in cells may decrease their sensitivity to some cytotoxic drugs. CYP1B1-activated prodrugs may therefore be useful for the treatment of cancers whose drug resistance is mediated by CYP1B1.
Furthermore, the CYP1B1 gene is highly polymorphic in cancer and several single nucleotide polymorphisms contained within the CYP1B1 gene have been identified that alter the expression and/or activity of the encoded protein. Of these, the CYP1B1*3 (4326C>G; L432V) allele has been characterized by both increased expression and enzyme kinetics of CYP1B1 toward several substrates as described by Sissung, et al. in Mol Cancer Ther., 7(1): 19-26, 2008 and references quoted therein. This finding indicates that not only CYP1B1 but the allelic variants of the enzyme may also contribute to prodrug activation and cancer targeting.
Prodrugs have been investigated as a means to lower the unwanted toxicity or some other negative attribute of a drug without loss of efficacy. A prodrug is a drug that has been chemically modified to render it inactive but that, subsequent to administration, is metabolized or otherwise converted to an active form of the drug in the body. The over-expression of CYP1B1 in primary tumours and metastatic disease compared to normal tissue offers a tremendous opportunity for the development of CYP1B1-activated prodrugs for targeted cancer therapy as reviewed by McFadyen et al., Mol Cancer Ther., 3(3), 363-71, 2004. Indeed, the discovery and development of CYP1B1-activated prodrugs for targeted cancer therapy is likely to offer significant pharmacological advantages over existing non-targeted cytochrome P450-activated prodrugs used clinically such as the prodrug alkylating agents cyclophosphamide, ifosfamide, dacarbazine, procarbazine which are activated by cytochrome P450s expressed in normal tissue as reviewed by Patterson L H and Murray G I in Curr Pharm Des., 8(15): 1335-47, 2002.
The human cytochrome P450 family contains 57 active isozymes, which function in normal metabolism, influence drug pharmacokinetics and effect negative outcomes in patients through drug-drug interactions. The cytochrome P450 isoenzymes metabolize approximately two thirds of known drugs in humans, with 80% of this attributable to five isozymes, namely CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 as described in Ortiz de Montellano, P R (ed.) Cytochrome P450: structure, mechanism, and biochemistry, Kluwer Academic/Plenum Publishers, New York, 2005.
Among the genes discovered by initiatives in the human genome project are CYP2R1, CYP2W1, CYP2S1, CYP2S1, CYP2U1 but the function, polymorphism and regulation of these genes are still to be fully elucidated as reviewed by Ingelman-Sundberg, M., Toxicol. Appl. Pharmacol., 207, 52-6, 2005. In addition to CYP1B1 a number of these cytochrome P450 oxidoreductases are extrahepatic and over-expressed in cancer. Several cytochrome P450s including CYP1B1, CYP2A/2B, CYP2F1, CYP2R1, CYP2U1, CYP3A5, CYP3A7, CYP4Z1, CYP26A1, and CYP 51 are present at a significantly higher level of intensity than in normal ovary as determined by immunohistochemistry and light microscopy, as described by Downie et al., Clin. Cancer Res., 11(20): 7369-75, 2005. Furthermore, using similar methods of detection in primary colorectal cancer, several cytochrome P450s, including CYP1B1, CYP2S1, CYP2U1, CYP3A5, and CYP51, are frequently over-expressed compared to normal colon as descried by Kumarakulasingham et al., Clin. Cancer Res., 11(10): 3758-65, 2005. In the same study several cytochrome P450s, including CYP1B1, CYP2A/2B, CYP2F1, CYP4V2, and CYP39, correlated with their presence in the primary tumour. CYP2W1 has also been shown to be over-expressed in colorectal cancer according to Elder et al., Eur. J. Cancer, 45(4): 705-12. CYP4Z1 is over-expressed in breast carcinoma is a gene associated with non-small cell lung cancer promotion and progression as described by Reiger et al., Cancer Res., 64(7): 2357-64, 2004 and Bankovic et al., Lung Cancer, 67(2): 151-9, 2010, respectively.
A major challenge in the field is elucidation of the function of human cytochrome P450s of so-called ‘orphan’ status with unknown substrate specificity as reviewed by Strak K and Guengerich F P in Drug Metab. Rev., 39(2-3): 627-37, 2007. A number of substrates are known for CYP1B1 few of which are specifically metabolised by the enzyme, for example 7-ethoxyresorufin undergoes oxidative de-ethylation when activated by all members of the CYP1 family, including CYP1A1, CYP1A2, and CYP1B1, as described by Chang T K and Waxman D J in Method Mol. Biol., 320, 85-90, 2006. A number of fluorgenic and luminogenic probe substrates are available to assess cytochrome P450 activity with high sensitivity but they exhibit broad specificity and as such are metabolised by a range of cytochrome P450 enzymes in the CYP1, CYP2, and CYP3 families. For example, Cali et al., Expert Opin. Drug Toxicol., 2(4): 62-45. 2006 describes the use of luminogenic substrates which couple to firefly luciferase luminescence in a technology called P450-Glo. Another example, is 7-ethoxycoumarin which undergoes cytochrome P450-catalyzed 7-ethoxycoumarin O-deethylation to release the highly fluorescent anion as described by Waxman D J and Change T K H in “The use of 7-ethoxycoumarin to monitor multiple enzymes in the human CYP1, CYP2, CYP3 families” in Methods in Molecular Biology, vol. 320, Cytochrome P450 Protocols, Second Edition, edited by Phillips I R and Shephard, E A, 2006.
Everett et al., Biochem. Pharmacol., 63, 1629-39, 2002 describe the reductive fragmentation of model indolequinone prodrugs by cytochrome P450 reductase (not to be confused with cytochrome P450s) in anoxia to release the 7-hydroxy-4-methylcoumarin anion. The model prodrug was non-fluorescent at the pre-selected emission wavelength and reductive fragmentation could be accurately measured by monitoring the production of the coumarin anion (λex=380 nm/λem=450 nm) using kinetic spectrofluorometry.
Interactions between a limited number of compounds (typically <100) and cytochrome P450s isozymes have been described but results from such studies are difficult to compare because of the differences in technologies, assay conditions and data analysis methods as described by Rendic, S. “Summary of information on human CYP enzymes: human P450 metabolism data” in Drug Metab. Rev., 34, 83-448, 2002. Many computational strategies have been advanced to generate predictive cytochrome P450 isozyme substrate activity models but these are limited by a lack of a single large, diverse data set of cytochrome P450 isozyme activities as described by Veith et al., Nature Biotechnology, 27, 1050-55, 2009. The authors describe the construction of cytochrome P450 bioactivity databases using quantitative high-throughput screening (HTS) with a bioluminescent enzyme substrate inhibition assay to screen 17,143 chemical compounds against five cytochrome P450 isozymes (CYP1A2, 2C9, 2C19, 2D6, and 3A4) expressed in normal tissues mainly the liver and responsible for so-called phase 1 metabolism of drugs. It was concluded that the database should aid in constructing and testing new predictive models for cytochrome P450 activity to aid early stage drug discovery efforts.
Jensen et al., J. Med. Chem., 50, 501-11, 2007 describe the methods for the in silico prediction of CYP2D6 and CYP3A4 inhibition based on a novel Gaussian Kernel weighted k-nearest neighbour (k-NN) algorithm based on Tanimoto similarity searches on extended connectivity fingerprints. The data set included modelling of 1153 and 1182 drug candidates tested for CYP2D6 and CYP3A4 inhibition in human liver microsomes. For CYP2D6, 82% of the classified test compounds were predicted to the correct class and CYP3A4, 88% of the classified test compounds were correctly classified.
Theoretically it may be possible to use cytochrome P450 HTS to build a large database of bioactivities for tumour and normal tissue cytochrome P450s and then develop a substrate prediction model as a basis for the design and synthesis of selective CYP1B1-activated prodrugs while screening out for pharmacological liabilities associated with Phase 1 metabolism by normal tissue cytochrome P450s. However, the reduction to practice is not obvious from prior art and has to be rationalised against prodrug structure and mechanism of conversion to the active drug when activated by tumour-expressing cytochrome P450s.
Utilization of so-called ‘trigger-linker-effector’ chemistry in prodrug design requires the activation of the trigger to initiate the fragmentation of a linker to release an effector (typically an active drug), the biological activity of which is masked in the prodrug form. The modular design of selective prodrugs targeted at tumour-expressing cytochrome P450s such as CYP1B1 require (1) the identification of selective trigger moieties, (2) the use of bio-stable linkers which fragment efficiently following trigger activation (usually by aromatic hydroxylation), and (3) suitable effectors or drugs which do not interfere with the efficiency of the triggering process.
CYP1B1 mRNA is expressed constitutively in all normal extrahepatic human tissues, though the protein is usually undetectable. In contrast, CYP1B1 protein is expressed at high levels in tumours. It is understood that for a large range of established or immortalized tumour cell lines (such as the MCF-7 breast cancer cells) originating from humans which have undergone significant passaging in vitro but does not constitutively express active CYP1B1 protein. Although CYP1B1 is not constitutively expressed in MCF-7 breast tumour cells it is possible to induce CYP1 enzyme expression both at the mRNA and protein level by treating with aryl hydrocarbon agonists such as the dioxin TCDD.
WO 99/40944 describes prodrugs that comprise a drug moiety bound to a carrier framework, the prodrug being described activated as though hydroxylation by CYP1B1 to release the drug moiety.